The desire to integrate data, voice, image, and video over high speed digital trunks has led to the development of a packet switching technique called cell relay or asynchronous transfer mode.
A typical fully-integrated voice and data network using digital trunk lines (e.g., T1, FT1, E1, T3, etc.) includes a number of geographically distant interconnect nodes. Each node acts as a cell exchanger for receiving and forwarding cell information to its destination. By the use of a variety of interface cards, each node is capable of interfacing to user generated voice and data streams, then segmenting and assembling the streams into a more efficient cell format for transmission over a closed network using digital lines. Each node is also capable of receiving data from other network nodes and forwarding that data through to other network nodes to its ultimate destination. All terminal nodes also provide the necessary interface cards required to reassemble the data contained in the cells into a standard user data stream format.
A typical modern node is capable of handling six classes of cell traffic, each class having different characteristics and different service requirements. The six classes of traffic include:
(1) High priority ("HP") for node-to-node control messages;
(2) Voice for pulse code or adaptive pulse code voice signals;
(3) Low speed statistical ("LSS") traffic sent at rates of less than 64 Kbps which includes statistical and deterministic full-period traffic;
(4) High speed deterministic ("HSD") traffic for full period voice or data at data rates exceeding 64 Kbps;
(5) Bursty data for point-to-point traffic generated by such sources as local area networks, bridges, routers or high speed packet switches within the cell network; and
(6) Multicast traffic which is of the same type as bursty data except that it is simultaneously broadcast from one source to many destinations (vs. point-to-point).
Each of these six traffic types are buffered at each network node in accordance with their particular sensitivities to network delay and cell loss. Cell loss may occur due to intermittent short term overload of network bandwidth and lack of adequate buffer capacity. For example, voice traffic is relatively delay sensitive and insensitive to occasional cell loss. In contrast, data traffic, such as file transfers, is relatively insensitive to delay but is data loss sensitive. High priority data is both delay and loss sensitive. To accommodate these differences, each class of traffic is typically placed in a preassigned queue, each with a different service priority. During periods of network traffic congestion, when network traffic demand exceeds the network's bandwidth capacity, servicing algorithms are typically employed to discriminate between traffic classes in order to allocate bandwidth. Delay is managed by properly sizing the queue depths and prioritizing transmission within a class based upon a measure of the time that a cell has been in the network as, for example, by use of time stamps and hop counts.
Even with these sophisticated queueing and service algorithms, congestion (due to excess arriving traffic) can occur. This congestion is typically divided into three categories: short-term, medium-term, and long-term. Short-term congestion, typically handled by discarding traffic at the queue, may be done haphazardly or preferably selectively by having cells marked with their "discard eligibility". Long-term congestion is controlled by admission policies that allocate resources (bandwidth and buffers) at the time a connection is established. If no resources are available, the connection is not allowed.
Medium-term congestion control has been an active topic of research during the last couple of years. Two types of techniques that have been researched are (1) open-loop control techniques involving no explicit feedback from the network in which congestion is controlled by smoothing the flow of input traffic and (2) closed-loop techniques that sense the level of congestion on the virtual circuit and control the in-flow of traffic based on feedback of congesting status information to the source terminal.
An example of a general rate regulation scheme for a bursty data source on a per virtual connection basis is described in a paper by K. Bala, et al., entitled, Congestion Control for High Speed Packet Switched Networks, published in the proceedings of the IEEE INFOCOM Jun. 5-7, 1990, pages 520-526. At the initial establishment of a virtual connection, a minimum amount of guaranteed bandwidth is allocated. The simplest system described uses the concept of a "leaky bucket" input rate controller that uses "tokens" and "spacers" to control the average data rate introduced into the packet switched network. Tokens arrive at the controller at a fixed rate. Each token corresponds to fixed number of bytes. The controller buffers the packet until enough tokens are collected for transmitting the entire packet. The token bucket has a fixed maximum capacity corresponding to the maximum packet burst duration. Tokens arriving to a full bucket are dropped. Thus, the system can handle different length packets which are transmitted without fragmentation. Peak rate control is accomplished by means of a spacer that introduces a suitable delay proportional to the length of the prior transmitted packet.
A given session on a virtual connection may last for long periods of time (up to hours). Bursty data sources are characterized by intermittent high data rate burst with significant spans of inactivity between bursts. Under these circumstances, the above described simplest system would result in underutilization of the bandwidth capacity of the system because of the prescribed safe bandwidth limit assigned to the virtual connection session.
Average bandwidth utilization efficiency is typically improved by introducing "colored" tokens, for example, green and red. Green tokens correspond to packets received for transmission that fall within the minimum guaranteed bandwidth protocol while the red tokens correspond to packet data received for transmission in excess of the guaranteed minimum rate. Intermediate nodes provide per trunk FIFO buffer service and use the colors associated with each packet for congestion control. In general, green packets are protected and passed along while red packets are discarded upon arrival whenever the chosen metric (usually queue lengths) for congestion threshold is exceeded. Even though discarding of packets implies retransmission of the lost packet data, the system is represented as improving the average utilization of bandwidth capacity.
An example of prior art feedback control is a window control method, described by K. K. Ramakrishnan and Raj Jain in an article entitled A Binary Feedback Scheme for Congestion Avoidance in Computer Networks, ACM Transactions on Computer Systems, Vol. 8, pages 158-181, May 1990. The window control method indirectly controls the effective network data rate by having the user adjust the window duration controlling the number of contiguous packets that can be transmitted at a given time.
For ISO Transport network architectures, each network layer packet has a header that is used for "housekeeping" functions, including routing. Within that header, a bit is assigned as a congestion indication bit. The packet source clears the congestion indication bit when a packet is originally transmitted from a source node end system. Each network router is equipped with congestion detection means that monitors the average queue length at the router at the time the packet arrives.
The average queue length is determined by the number of packets that are queued and in service, averaged over an interval. This interval corresponds to the duration of the sum of the last busy plus idle cycle duration plus the busy interval of the current cycle. When certain average queue length threshold conditions are met at any router, the congestion indication bit is set and forwarded with its associated packet. Once set, the congestion bit remains set even though it subsequently passes through routers with queue lengths not exceeding threshold.
At the destination end system (user transport entity) an acknowledgement message is generated if no error is detected and is sent together with the congestion indication bit status associated with the acknowledged packet. The user transport entity updates the window size based on the number of packet congestion bits set during the last transmission window used. If at least 50% of the bits are set, the window size is reduced from its current value. Otherwise, it is increased. In order to avoid instability in this feedback control system, updating of the user's transparent entity window size occurs only after a time period corresponding to the duration of the last two windows. This delay allows enough time to see the effects of the last window change. In order to promote "fairness" of access by all network users and to achieve stable operation, the window size adjustment algorithm provides for increasing window size by linear increments and for decreasing window size by a multiplicative factor.
In summary, Ramakrishnan and Jain describe a system using window control at the ISO Transport layer using window duration control rather than rate control. Rate control is indirectly controlled by the limiting actions of acknowledgements and window length. Because transmission rate is a direct measure of bandwidth, better short term control of this system resource can be obtained by direct rate control.
C. A. Cooper and K. I. Park in an article entitled Toward a Broadband Congestion Control Strategy, IEEE Network Magazine, May 1990, p. 18-23, discuss a congestion control system for variable bit rate traffic using an ATM based fixed length cell switching network which may have elements of both open-loop and closed-loop control. The authors describe a system carrying different classes of service in which a cell rate is assigned to variable bit rate data that is less than the available peak cell rate so as to provide capacity for statistical multiplexing of data. A new connection is admitted only if sufficient network resources are available to support that connection. Cells offered to the network in excess of the assigned rate are immediately discarded because, in the opinion of the authors, the tagging of floating cells for possible subsequent discard appears to yield no substantial advantage. Traffic enforcement is applied on the ingress direction of each virtual circuit. A leaky bucket or credit manager algorithm is used to control the rate.
The Cooper article refers to a reactive (feedback) control method that combines cell tagging with explicit congestion notification ("ECN"). When an asynchronous transfer mode ("ATM") switch detects congestion on a given route, it sends an ECN message over the variable rate virtual circuits on that route to the serving ATM switches which may, in turn, relay these ECN messages to the involved customer premises node. The serving ATM switches operate at one of two preset thresholds for rate enforcement on each variable bit rate virtual circuit using ECN. The lower threshold is used during congestion. Additionally, the ECN message may be transported between network nodes possibly using a separate virtual circuit, and possibly using a bit in the ATM cell header.